(1) Field of the Invention
The invention relates to the fabrication of semiconductor circuits and more specifically to a method of creating bit-line and charge node contacts for embedded DRAM circuitry.
(2) Description of the Prior Art
Self-alignment is a technique in which multiple levels of regions on the wafer are formed using a single mask, thereby eliminating the alignment tolerances required by additional masks. This approach has been used more often as circuit dimensions decrease. There are many examples of this technique, one of the earliest and most widely used is the self-aligned source and drain implant to the poly gate. Self-aligned contacts are often used in memory cells where contacts are limited only by the spacers and field oxide bird's beak or a contact window-landing pad. Therefore, the mask contact window can be oversized and no contact borders are needed, resulting in significant space savings.
As transistor dimensions approached 1 um, the conventional contact structures used up to that point began to limit device performance in several ways. First it was not possible to limit the contact resistance if the contact hole was also of minimum size while problems with cleaning the contact hole also arose. In addition, the area of the source/drain region could not be minimized because the contact hole had to be aligned to these regions with a separate masking step and extra area had to be allocated for misalignment. The technique of producing several small, uniform sized contact holes was also used, the reason for this being that if all contact holes are of uniform size, they are more likely to clear simultaneously during the etching process. The problem with this latter approach is that the full width of the source/drain region is not available for the contact structure. As a result, the device contact resistance was proportionally larger than it would have been in a device having minimum width.
A variety of alternative contact structures have been investigated in an effort to alleviate this problem. Among these are self-aligned salicides on the source/drain region, elevated source/drain regions, buried-oxide MOS contacts and selectively deposited layers of metal in the contact holes.
As the density of circuit components contained within a semiconductor die has increased and the circuit components have decreased in size and are spaced closer together, it has become increasingly difficult to access selectively a particular region of the silicon wafer through the various layers that are typically superimposed on the surface of the silicon wafer without undesired interference with other active regions.
It is especially important to have a technology that can etch openings that have essentially vertical walls, most notably when the openings are to extend deeply into the surface layers. Special care must also be taken to insure that the profile of the lower section or bottom of the opening resembles a straight line in order to reduce thickness difference in the underlying layers. To this end, it is critically important to select a stop layer (that has a restraining influence on the etching process) within the semiconductor structure that enhances the linearity or straight-line profile of the bottom of the etched hole.
Additionally, to tolerate some misalignment in the masks used to define such openings, it is advantageous to provide protection to regions that need isolation but that inadvertently lie partially in the path of the projected opening. To this end it is sometimes the practice to surround such regions with a layer of material that resists etching by the process being used to form the openings. Accordingly, a technology that provides the desired results will need an appropriate choice both in the materials used in the layers and the particular etching process used with the materials chosen.
Dry etching, such as plasma etching and reactive ion etching, has become the technology of choice in patterning various layers that are formed over a silicon wafer as it is processed to form therein high density integrated circuit devices. This is because it is a process that not only can be highly selective in the materials it etches, but also highly anisotropic. This makes possible etching with nearly vertical sidewalls.
Basically, in plasma etching as used in the manufacturing of silicon integrated devices, a silicon wafer on whose surface have been deposited various layers, is positioned on a first electrode in a chamber that also includes a second electrode spaced opposite the first. As a gaseous medium that consists of one or more gasses is flowed through the chamber, an r-f voltage, which may include components at different frequencies, is applied between the two electrodes to create a discharge that ionizes the gaseous medium and that forms a plasma that etches the wafer. By appropriate choice of the gasses of the gaseous medium and the parameters of the discharge, selective and anisotropic etching is achieved.
While elaborate theories have been developed to explain the plasma process, in practice most such processes have been developed largely by experimentation involving trial and error of the relatively poor predictability of results otherwise.
Moreover, because of the number of variables involved and because most etching processes depend critically not only on the particular materials to be etched but also on the desired selectivity and anisotropy, such experimentation can be time consuming while success often depends on chance.
The present invention teaches an improved method of creating a borderless pedestal that can be used for bit-line contact and charge-node contact.
U.S. Pat. No. 5,631,179 (Sung et al.) shows self aligned contacts (SAC) for a flash memory.
U.S. Pat. No. 5,712,201 (Lee et al.) teaches a SAC poly plug process for a DRAM.
U.S. Pat. No. 5,702,990 (Jost et al.) teaches a poly plug contact for a bit line.
U.S. Pat. No. 5,700,706 (Juengling) discloses a process of SAC poly plug contacts.
U.S. Pat. No. 5,488,001 (Figura et al.) teaches SAC plug processes having a barrier layer 43 for node contacts.